For Outlook 2019

Before starting with cleaning up, you'll first need to know how “dirty” your mailbox really is. To get an
overview of the total size of your mailbox and each individual folder you can open the Folder Size dialog;
1. Click File
2. Mailbox Cleanup

3. Click the button: View Mailbox Size…

In the window that is being displayed now, you’ll see an overview of your mailbox folders and their sizes..

Empty the Deleted Items folder – Empty the Deleted Items folder frequently to make sure you aren’t keeping messages you don’t need.
Empty the Junk Email folder – Periodically empty the Junk Email folder to save your inbox space for messages you actually want.
Store attachments outside your mailbox – As attachments accumulate over time, they can take up a lot of space. For attachments you want to keep, consider saving them to a team site, to OneDrive, or to folders on your computer.

How to clean up my mailboxMTE.pdf

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